The present invention relates to electrical discharge machining processes for cylindrical works of the type designed to give, by electrical discharge, a satiny finish of a predetermined surface roughness to the outer surface of a cylindrical work, and more particularly, the invention relates to an electrical discharge machining process capable of preventing the occurrence of a stepped portion on the boundary portion between the working areas of the adjacent electrodes due to the difference in the number of passages of electrodes between the electrodes during the electrical discharge machining.
In the past, the method of throwing hard metal particles, e.g., shot or grit, at the polished surface of strip rolling rolls, particularly cold rolling rolls or the like, to produce impressions on the roll surface, has been used as a means of giving a satin-like finish to the surface of such roll, and recently attempts have been made to replace this type of process by electrical discharge machining. As is well known, electrical discharge machining is a process in which an insulating liquid such as kerosene is placed in a small discharge gap between an electrode and a work piece and a pulse wave voltage is applied periodically across the electrode and the work piece, thus machining the surface of the work piece. By repeatedly performing this electrical discharge machining at the surface of a roll while rotating the roll in the circumferential direction and also gradually moving the electrode along the axis of rotation of the roll, it is possible to continuously give a spiral satiny finish to the roll surface, and thereby to produce spark eroded impressions all over the roll surface. This is the known method of giving a uniform satiny finish to the surface of a roll. The satin finish produced in this way has many advantages; that is, not only is the surface roughness more marked and the shape in excellent trim as compared with the mechanical impressions produced by the throwing of metal particles, but also the shape is not affected by the manufacturing method, hardness, etc., of the roll, the metal structure in the roll surface being hardened by the electrical discharge, thus making the roll best suited for rolling purposes and so on.
Also in the past, an electrical discharge dull finishing process has been proposed in which a roll is rotated at a constant speed and a head column (a head and a column are considered as a unit and the unit is hereinafter referred to as a head column) having an electrode is traversely fed repeatedly by an amount equal to the working width of the roll along its axis of rotation. However, this method has the disadvantage of requiring an exceedingly large working time and there is also an inconvenience that the method cannot, as such, be put to practical use.
As a result, more recently, electrical discharge dull finishing processes have been proposed in which a multi-piece electrode is used to reduce the working time. However, the processes which have been proposed so far are so designed that a roll is rotated at a constant speed and split electrodes are very inacurately fed transversely to thereby give a desired finish to the entire roll surface, and consequently the measures taken to deal with the difficulty at the boundary portions between the working areas of the electrodes are extremely inadequate. Actual operations performed with this type of process showed that very small steps or stepped portions were produced on the boundary portions. While the experiment also showed that the magnitude of such steps was very small, i.e., on the order of several microns in the case of a surface roughness of 18 .mu.Rz, when the thusly dull finished roll is used actually in the final rolling of sheet steel, the effect of such steps cannot be ignored, and the rolled sheet steel products will be rejected.
The cause of such steps is the fact that an inacurate transverse feeding of the electrodes causes a difference in the number of passages of electrodes between the boundary portion of the working areas and other portions, and consequently the depth of spark erosion is not the same at all the portions.